1. Field of the Invention
This invention relates to the improvement of a semiconductor device having a charge transfer device, bipolar transistor and MOSFET formed on the same chip.
2. Description of the Related Art
Conventionally, a CCD which is a typical example of a charge transfer device is formed together with N-channel MOSFETs on one chip. Such N-channel MOSFETs formed together with the CCD constitute a CCD driving circuit, sample-hold circuit, output circuit, clock driver circuit or the like In most cases, a power source voltage for the circuits using the MOSFETs is set at 12 V or 9 V.
Recently, a tendency for MOSFETs to be formed in a CMOS configuration becomes greater as the power consumption is reduced. At the same time of forming the MOSFETs in the CMOS configuration, the power source voltage tends to be set to 5 V in order to set the power source voltage equal to that used for a bipolar IC formed in a different chip.
However, when the power source voltage is set to as low as 5 V, the linearity of the input-output characteristics of a linear amplifier (operation amplifier) constituting an output circuit and a sample-hold circuit constituted by MOSFETs will be degraded in comparison with the case of using the power source voltage of 12 V or 9 V. When the linearity of the output of the operation amplifier is degraded, the characteristic is determined to be defective at the time of testing the wafer, thereby lowering the yield.
Further, the CCD delay line only has a function of delaying a signal, and in many cases, a circuit for processing the delayed signal is formed on a bipolar IC of a different chip.
If the sample-hold circuit and the operation amplifier such as an output circuit are replaced by a bipolar transistor which has a better linearity and higher operation speed than the MOSFET at the time of low power source voltage operation, the linearity o the input-output characteristic can be improved so as to enhance the yield and attain a high performance. Further, if the bipolar IC for processing the CCD signal is formed on the same chip in a monolithic configuration, the occupied space can be reduced, the manufacturing cost can be lowered, the function can be enhanced and the system can be made simple.
However, if charges of the transfer clock for the charge transfer of the CCD leak into the base region of the bipolar transistor, the bipolar transistor section cannot be operated properly. Therefore, it has been difficult to dispose the CCD and the bipolar transistor on the same chip. This is because the bipolar transistor having a large current amplification factor (h.sub.fe) is significantly influenced by the leaked charges of the transfer clock and the operation characteristic thereof is deteriorated so that the reliability thereof cannot be made so high as that of an acceptable device.